In 1880 the American stage mentalist Washington Irving Bishop published a book revealing how mediums would use secret codes as the trick for their clairvoyant readings.[84] The Seybert Commission was a group of faculty at the University of Pennsylvania who in 1884–1887 exposed fraudulent mediums such as Pierre L. O. A. Keeler and Henry Slade.[85] The Fox sisters confessed to fraud in 1888. Margaret Fox revealed that she and her sister had produced the "spirit" rappings by cracking their toe joints.[86]